Frames: Problem Solving I

A two-force member is a straight element subjected only to forces at its two ends, with forces equal in magnitude but opposite in direction, causing pure tension or compression. A multi-force member experiences more than two forces distributed along its length, including external loads, reaction forces at pin joints, and cable forces, resulting in complex stress distribution.

For a frictionless pulley system, the load weight balances the tension in the cables. In the lower pulley section, each cable carries an upward tension equal to half the total load. For the upper pulley, the vertical cable tension remains the same as the lower section since it is part of the continuous cable system.

Force equilibrium conditions require that the sum of horizontal and vertical forces equals zero at each joint. Moment equilibrium conditions require that the sum of moments about a point equals zero. These conditions are applied systematically to free-body diagrams of each structural member to solve for unknown reaction forces.

The force FBD in member BD is resolved into horizontal and vertical components using a slope triangle, which relates the member's geometry to its force components. The moment equilibrium condition at joint A yields the magnitude of FBD. Substituting known dimensions and pulley radius values determines FBD as 50 kN in this example.

Reaction forces depend on load magnitude and structural geometry. In this jib crane example, the horizontal reaction force at joint A is 40 kN, while the vertical reaction force is -20 kN. At joint D, the horizontal reaction is -30 kN and the vertical reaction is 40 kN, determined by applying force equilibrium conditions.

The lower pulley section directly supports the external load, establishing the cable tension value. This tension value propagates through the continuous cable system to the upper pulley section. Determining lower pulley tension first provides known force values needed to solve for reaction forces at pin joints in the frame structure.

Free-body diagrams isolate each structural member and show all forces acting on it, including external loads, cable tensions, and reaction forces at pin joints. Applying force and moment equilibrium conditions to these diagrams systematically yields unknown reaction forces. This method using method of joints problem solving principles allows solving complex frame structures step by step.